Study Concludes That Smaller Capacity PHEVs Would Be a “Robust” Option for Minimizing Fuel Consumption, Cost and GHG Emissions

26 February 2009

Best vehicle choice for minimum fuel consumption, cost, or greenhouse gas emissions as a function of distance driven between charges across sensitivity scenarios. Shiau et al. (2009) Click to enlarge.

A team of researchers from Carnegie Mellon University has analyzed the impact of plug-in hybrid electric vehicle (PHEV) battery pack size on fuel consumption, cost and greenhouse gas emissions over a range of charging frequencies (distance traveled between charges). The study will appear in an upcoming issue of the journal Energy Policy.

When charged frequently (every 20 miles or less), using average US electricity, small capacity (i.e., lower all-electric range) plug-in hybrid electric vehicles (PHEVs) are less expensive operationally and release fewer greenhouse gases (GHGs) than hybrid-electric (HEVs) or conventional vehicles, according to the study’s findings.

“Larger battery packs allow drivers to go longer distances on electric power. But batteries are heavy and expensive. Over a range of scenarios—including fluctuating gas prices, new battery technologies or high taxes on carbon dioxide emissions—plug-ins with small battery packs are economically competitive with ordinary hybrid and conventional vehicles for drivers who charge frequently.”

—Prof. Jeremy Michalek

For moderate charging intervals of 20-100 miles, PHEVs release fewer GHGs, but HEVs are more cost-effective, the study found. Large-capacity PHEVs—sized for 40 or
more miles of electric-only travel—are not cost-effective in any scenario, according to the findings, although they could minimize GHG emissions for some drivers and provide potential to shift air pollutant emissions away from population centers.

The study, led by Assistant Professor Jeremy Michalek, indicated that the impacts of increased battery weight from larger packs on charge-depleting (CD) mode electrical efficiency and charge-sustaining (CS) mode fuel economy are measurable, with about a 10% increase in Wh/kg and an 8% increase in gallons per mile when moving from a PHEV7 to a PHEV60. This implies, the researchers said, that the additional weight of a PHEV60 results in a 10% increase in operation-related costs and greenhouse gas emissions per mile relative to a PHEV7 for drivers who charge frequently (every 7 miles or less).

The best choice of PHEV battery capacity depends critically on the distance that the vehicle will be driven between charges. Our results suggest that for urban driving conditions and frequent charges every 10 miles or less, a low-capacity PHEV sized with an AER of about 7 miles would be a robust choice for minimizing gasoline
consumption, cost, and greenhouse gas emissions.

An increase in gas price, a decrease in the cost of usable battery capacity, or a carbon tax combined with low carbon electricity generation would increase PHEV cost effectiveness for a wide range of drivers. In contrast, a battery technology that increases specific energy would not affect net cost and GHG emissions significantly, and a $100 per ton carbon tax without a corresponding drop in carbon intensity of electricity generation would not make PHEVs significantly more competitive.

These results suggest that research on PHEV battery technology improvements would be better targeted toward cost reduction than improvement of specific energy, and the effect of carbon taxes on the PHEV market will depend on their effect on the electricity generation mix, such as encouraging renewables, carbon capture and sequestration, and nuclear.

...The dominance of the small-capacity PHEV over larger-capacity PHEVs across the wide range of scenarios examined in this study suggests that government incentives
designed to increase adoption of PHEVs may be best targeted toward adoption of small capacity
PHEVs by urban drivers who are able to charge frequently.

—Shiau et al. (2009)

The study, which was funded by the National Science Foundation and the Teresa Heinz Scholars for Environmental Research Program, points out that targeting drivers with the potential to charge frequently would not limit plug-ins to a boutique market: nearly 50% of US passenger vehicle miles are traveled by vehicles driving less than 20 miles per day.

The researchers used the split drivetrain configuration of a 2004 Prius as the baseline HEV, and examined PHEV versions of it sized for 7, 20, 40, and 60 miles (11, 32, 64 and 96 km) of all-electric range (AER) with comparable performance characteristics.

For simplicity, they assumed an operating strategy in which the PHEVs run entirely on electric power in the charge-depleting (CD) range (i.e., not blended mode), and then switch to operate like an HEV in the charge-sustaining (CS) range. The battery packs used Saft Li-ion cells with 6 Ah capacity and nominal output voltage of 3.6V. The researchers used Argonne National Laboratory’s Powertrain System Analysis Toolkit (PSAT) to model and examine design tradeoffs between battery capacity and PHEV benefits.

The researchers modified the control strategy so that the PHEVs operated in electric-only
charge-depleting mode until the battery reaches 35% SOC, after which time the vehicle switches to CS-mode and operates like a Toyota Prius, using the split control strategy with a target SOC of 35% and SOC operating range of 30-40%.

Comments

For many families I'm guessing that owning both a small, all-electric commuter car for daily use plus a regular hybrid or other high-mileage ICE car for longer trips would make a lot of sense. PHEVs are going to be expensive and haul a lot of dead weight around (the range extender) most of the time.

This is a very academic and theoretical minded look at the issue that misses the fundamental point of a larger battery in vehicles that plug-in. Higher energy in a larger battery does displace more fuel directly, but what it really does is provide sufficient EV power, so that ALL ELECTRIC RANGE (AER) is actually possible.

The researchers here take the easy way out and make the gross simplification:

"for simplicity and fair comparisons we restrict attention to the range-extended PHEVs that run entirely on electrical power in the charge-depleting range and switch to operate like an HEV in the charge-sustaining range"

They simply take a Prius and simulate it as if it could drive the first 7,10,or 20 miles on electric power alone. But a small battery they end up advocating for could not actually drive the vehicle entirely on electric power alone, and has no real world AER. Hence, this is a fundamentally flawed study.

For instance, in order for a Prius to drive preferentially on electric power alone, it would need a 70kW or so battery pack, since the Prius car has approximately 70 kW of total usable power with the engine off. The C-rating for an energy-type battery pack is typically no higher than 5 or so, meaning that it can provide power at at best approximately 5 times its 1 hour rated capacity. So a 70 kW battery is going to have about 14 kWHr of energy on board. Approximately 60% is available for EV discharge within safe and durable limits, so we have a usable 8.4 kWHr of battery energy. A car like the Prius will use approximnately 250 Watt hrs per mile, so it would have an EV range of about 8.4/.25 or about 34 miles. If the vehicle had less energy, it would not have sufficient EV power, and the AER (All Electric Range)that the academics toss around becomes meaningless. The minimum real world AER for a Prius is about 34 miles. For vehicles with less energy on board the engine will be going on and off a lot and the kind of analysis that professors did misses the boat entirely.

Other things beyond just the power rating of the battery constrain usability of EV power in a plug-in hybrid. The drive motor power, the electronics, the gearing the thermals systems, etc, all have to be rated for 70 kW in order to make a Prius a vehicle that has a meaningful AER. Just the other day, the Seattle fleet of Plug-in Prius conversions demonstrated this exactly, getting no better mpg than the base Prius: http://www.autobloggreen.com/2009/02/24/disappointing-results-just-51-mpg-for-converted-phev-priuses/

A vehicle that is configured to have sufficient EV power to have a real world electric driving is known as an E-REV, like the VOLT. The benefits of E-REVs relative to PHEVs without a large battery and subsequently without AER capability (like the Plug-in Prius) was published in the GCC last year: http://www.greencarcongress.com/2008/02/gm-study-shows/comments/page/2/

One of the problems with E-REVs and PHEVs is that there is lot of hype and hope and positioning among opinion leaders, yet very few such vehicles actually exist, and very few people have actually driven one over a period of time and understands the real limitations, constraints and balances that need to be considered. The best academic work at this time is more likely to be hands-on the precious few fleets that exist.

frankbank - while I find your analysis of the battery sizing issue for PHEVs interesting, I have to correct you regarding the Seattle fleet study.

While the overall fleet economy for their PHEVs is rather dismal, you have to dig deeper in the report which was linked somewhere in the comments: http://seattletimes.nwsource.com/ABPub/2009/02/20/2008768175.pdf

What you find is that about half of the trip miles driven were driven in CD mode. The other half of the trip miles were driven in either combo CD/CS or CS mode.

Moreover, if you look at the overall fuel economy compared to the CS mode fuel economy - while not impressive to what most Prius drivers get, it was a decent amount better, about 20% better.

What is also interesting is how much driving aggressiveness affects fuel economy. They rate driving aggressiveness on a scale of 0-10. When driven between 0-2, fuel economy averaged 100mpg. When driven between 2-4, fuel economy averaged less than 50mpg. When driven between 4-6, fuel economy averaged less than 40.

I would have thaught that what you want is enough battery power to do all urban driving on E power at least.
This will generate huge pollution benefits for the community and some decent fuel savings for the driver.
For short runs, the car should run on e power anyway.

I think focusing on AER is going to nedlessly postpone the adoption of very useful, shorter range PHEVs as people wait for huge batteries.

I think the reason why PHEV's will become popular is because except for the battery technology and the recharging connection, they use the same technology found on the Toyota and Ford HEV systems. This means we could start seeing PHEV's from an official source start appearing in the market probably by fall 2010, using either Li-On, zinc air or eventually carbon nanotube ultracapacitor battery packs, which would allow the PHEV to run in all-electric mode up to 43-49 miles (70-80 km) on a full charge before it switches back to standard hybrid drive mode.

Yeah... Let's remember the Global Warming scenarios that have turned out to be utterly falsified. Now there's the "Global Depression Crisis..." Except folks, you can only bullshite them a couple times before they say... "Um, you lied to us before, you're lying now."

When people get used to the plug-in paradigm, they're going to want an all-electric range that matches their normal daily usage. So 20 miles might be great for drivers who live within 10 miles of work but drivers who go 25 miles each way are going to want at least 50 miles range. Charging stations at work will help immensely and will be likely be demanded by many employees. They will begrudge those times when the gasoline engine kicks in because they know that means they're that much closer to having to stop at a gas station.

All of us now are used to regular gas stops and paying whatever the oil industry decides they can squeeze from us. As a society, were so accustomed to it that it's hard to imagine alternatives. An example is the vast network of EV charging stations many envision to exactly replace the role of today's gasoline filling stations. I say there will be relatively few such stations and they will account for less than 10% of EV charging.

In 10-20 years there'll be a lot more appreciation for being able to avoid filling/charging station trips and for being more free from big oil's whims and greed. 90% of the time EVs and PHEVs will be charged at home or the workplace.

I drive 10 miles round trip 3 days a week and 22 round trip 2 days. A few days a month I may drive 100 miles round trip. A PHEV with 10 mile all electric range would still cut my gas usage substanially even with no charging facility at work. If it meant the battery were affordable, great. 20 miles would be fabulous.

The primary goal of PHEV's is all-electric range of 60 to 70 km (37 to 43 miles) on a fully-charged battery pack. Given that most commuting is under 15 miles in range a PHEV version of the Prius or Ford Fusion will need a lot less fillups if you do most commute driving.

Mind you, I'd love to see Ford reduce the size of their HEV drivetrain so it fits in the current Ford Fiesta, though.

From the perspective of most electric utility companies, mass adoption of 30+ miles range all-electric cars may not be a viable option in the near future (next 10 years). First many new electric power plants would need to be built, transmission lines and transformers upgraded.

The shorter range PHEVs (10-20 miles) should be the focus until much higher energy density batteries are available at a reasonable price. It will take some time.

PHEVs with max electric-only speed of only up to 70 kmph (43 mph - for urban driving only) stand much better chance of commercial succes.
Reasons: e-motors for highway capable speeds (110-120 kmph) need to be much more powerful, and are currently much more expensive. The batteries would need to provide more power, it's critical for small battery packs. Plus added weight and more expensive electronics.

In 10-15 years many patents will expire which will further accelerate PHEV adoption.

It might help to have car rentals with PHEV/EV for rent. If you are having your car repaired, you can get one with 50 mile range and try it out. Recharge it in your garage with 110v over night and you might find that it could make a good second car.

For instance, in order for a Prius to drive preferentially on electric power alone, it would need a 70kW or so battery pack, since the Prius car has approximately 70 kW of total usable power with the engine off. The C-rating for an energy-type battery pack is typically no higher than 5 or so, meaning that it can provide power at at best approximately 5 times its 1 hour rated capacity.

The SafT 176065 cell is rated at 65 A max pulse discharge. That's 14.5 C; the max continuous discharge is 12.5 C. Using 14.5 C as the limit to achieve 70 kW, the battery pack only needs to be 5 kWh; at 12.5 C, it's 5.6 kWh. The Prius uses about 200 Wh/mi, so 65% of the smaller battery would yield about 15 miles AER.

Note that the current battery in the Prius is only about 1 kWh and some high-power Li-ion chemistries could reduce this substantially, so that battery capacity could probably be cut to under 3 kWh.

Congress is now the holdup here. The rules for PHEV tax credits set minimum battery capacities to qualify, rather than minimum AER. This appears to be intended to block small companies like Aptera and Venture Vehicles from qualifying. It's time to eliminate the welfare handouts to the major corporations and change the system to reward what we need: displacing liquid fuels.

MG, no new generating capacity would be required until very high PHEV/EV penetration is achieved so long as they charge at night; the US grid has ample excess capacity during off-peak hours. All we'd need is the fuel to run it. Short-range PHEVs recharging during the day would add to peak loads, so some adjustment would be required then.

This is interesting. The case I try to make for 10-20 mile range on electricity alone has more to do with its effect upon land-use and development. We simply drive too much and too far. Households with a car that may go 10 miles on cheap electricity (rooftop photovoltiac) will drive less. In time, more routine trips can be made without having to drive. Walking, bicycling can be safer and convenient and mass transit more practical to arrange.

Or, we can live in a future world like Tom Cruise in "Minority Report", where in sausage-shaped hypercars magnetically glued to 'verticle' off-ramps on the outside walls of high-rise condos or leading to gigantic unsupported freeways hundreds of feet off the ground, we speak derisively about "the Sprawl" and escape from Futurecrime Authority forces in completely computer controlled cars may not be possible. Thanks a lot, Spielberg!

I think people may see that they can have an EV in their garage that is just a car conversion to electric. It passes all crash tests in place for the year the car was made and can go at least 50 miles at 45 mph. The car can cruise at 70 mph and can drive on the the roads just like the car that was converted. If we can get these cars charged with solar panels on the roof...great. If not, we have part of the solution to the problem of imported oil from countries that fund terrorism.

The Saft cell you refer to is not a high energy cell, and it's design is for high power- A Crate of 14.5 reinforces that. Looking a the practial details, this cell has a only a usable 3.5 Ahr. To make your low AER Prius with sufficent electric power for 70KW, you indicated that you would need a usable 5 kWhrs worth of cells. Your 5 kWhrs pack would need to have about 500 of these cells, with 500 intercell interconnects and 500 channels of cell monitoring and/or control.

By the way, this is a Lithium Cobalt chemistry and is intended for military use. By the time you were done with the pack it would be more expensive than a pack made of cells designed with a safer automotive usage chemistry, and in the right cell size.

This further supports the idea that the academic study didn't comprehend the practical nature of plug in vehicles when they argued that a low AER was a good thing for cost reasons.

What is the average of KWH's used per mile by an EV ?
If they are all different what is the range?

What is the average charging in KWH's for a full charge?

Purpose of questions...
I am attempting to evaluate the EV operating cost vs gasoline and calculate the CO2 reduction if charged from Grid Electricity vs. Gasoline...
I know renewable is the way to go...
however the expense for a "Whole House Solar System" is still making this alternative prohibitive for most...